Thin film bulk acoustic resonator with lateral excitation of semi-lambda dielectric layer

Abstract

An acoustic resonator device and a filter are disclosed. An acoustic resonator device includes: a substrate having a surface; a single crystal piezoelectric plate having a front side and a back side, the back side attached to a surface of the substrate, except for a portion of the piezoelectric plate not attached to the surface of the substrate, where the portion of the piezoelectric plate forms a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front side of the single crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The piezoelectric plate and the IDT are configured such that a radio frequency signal applied to the IDT excites a shear primary acoustic mode in the diaphragm. And a half lambda dielectric layer formed on one of the front surface and the back surface of the piezoelectric plate.

Description

[0001] This application is a divisional application of the Chinese invention patent application "Thin film bulk acoustic resonator with lateral excitation of a half-λ dielectric layer" with an application date of March 16, 2020 and application number 202080018468.7. Technical Field

[0002] The present disclosure relates to radio frequency filters using acoustic wave resonators, and more particularly to filters for use in communication devices. Background Art

[0003] Radio frequency (RF) filters are two-terminal devices that are configured to pass some frequencies and block other frequencies, where "pass" means transmission with relatively low signal loss, and "block" means blocking or substantially attenuating. The range of frequencies passed by a filter is called the "passband" of the filter. The range of frequencies blocked by such a filter is called the "stopband" of the filter. Typical RF filters have at least one passband and at least one stopband. The specific requirements of the passband or stopband depend on the specific application. For example, a "passband" can be defined as a frequency range where the insertion loss of the filter is better than a defined value such as 1dB, 2dB, or 3dB. A "stopband" can be defined as a frequency range where the rejection of the filter is greater than a defined value, such as 20dB, 30dB, 40dB, or more, depending on the specific application.

[0004] RF filters are used in communication systems that transmit information over wireless links. For example, RF filters can be found in the RF front end of cellular base stations, mobile phones and computing devices, satellite transceivers and ground stations, Internet of Things (IoT) devices, laptops and tablets, fixed-point radio links, and other communication systems. RF filters are also used in radar and electronic and information warfare systems.

[0005] RF filters typically require many design trade-offs to achieve the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size, and cost for each specific application. Specific design and manufacturing methods and enhancements can benefit one or several of these requirements simultaneously.

[0006] Enhancements in the performance of RF filters in wireless systems can have a wide range of impacts on system performance. Improvements in RF filters can lead to improvements in system performance, such as larger unit size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be achieved individually or in combination at various levels of the wireless system, such as at the RF module, RF transceiver, mobile or fixed subsystem, or network level.

[0007] To obtain a wider communication channel bandwidth, it is necessary to use a higher frequency communication band. TM The LTE (Long Term Evolution) specification defines frequency bands between 3.3 GHz and 5.9 GHz. Some of these bands are currently unused. Future wireless communication proposals include millimeter wave communication bands, which have frequencies up to 28 GHz.

[0008] High performance RF filters used in current communication systems typically include acoustic wave resonators, which include surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR) and other types of acoustic resonators. However, these existing technologies are not well suited for use at higher frequencies, which are proposed to be used in future communication networks. Summary of the invention

[0009] The present invention relates to an acoustic wave resonator device, comprising: a substrate having a surface; a single crystal piezoelectric plate having a front side and a back side, the back side being attached to the surface of the substrate, except for a portion of the piezoelectric plate not being attached to the surface of the substrate, wherein the portion of the piezoelectric plate forms a diaphragm, the diaphragm spanning a cavity in the substrate; an interdigital transducer (IDT) formed on the front side of the single crystal piezoelectric plate such that the interlaced fingers of the IDT are arranged on the diaphragm, the piezoelectric plate and the IDT being configured such that a radio frequency signal applied to the IDT excites a shear main acoustic mode in the diaphragm; and a half lambda dielectric layer formed on one of the front side and the back side of the piezoelectric plate.

[0010] The thickness of the piezoelectric plate ts and the thickness of the dielectric layer td are defined as follows: 2ts =

[0011] λ 0,s , and 0.85λ 0,d ≤2td≤1.15λ 0,d , where λ 0,s is the wavelength of the fundamental shear bulk acoustic resonance in the piezoelectric plate, and λ 0,d is the wavelength of the fundamental shear bulk acoustic resonance in the dielectric layer.

[0012] Wherein, the dielectric layer is one or more of SiO2, Si3N4, Al2O3 and AlN.

[0013] The piezoelectric plate is lithium niobate, the dielectric layer is SiO2, and the thickness ts of the piezoelectric plate and the thickness td of the dielectric layer are defined by the following relationship: 0.875ts≤td≤1.25ts.

[0014] The frequency temperature coefficient of the acoustic wave resonator device is between -32 ppm / °C and -42 ppm / °C at the resonant frequency, and between -20 ppm / °C and -36 ppm / °C at the anti-resonant frequency.

[0015] The present invention also relates to a filter device, comprising: a substrate; a piezoelectric plate having parallel front and back surfaces and a thickness of ts, the back surface of which is attached to the substrate; a conductor pattern formed on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a corresponding plurality of resonators, the plurality of resonators comprising parallel resonators and series resonators, the interleaved fingers of each of the plurality of IDTs being disposed on a corresponding portion of the piezoelectric plate, the corresponding portion of the piezoelectric plate being suspended above one or more cavities formed in the substrate; a first dielectric layer having a thickness of tds, deposited between the fingers of the series resonators; and a second dielectric layer having a thickness of tdp, deposited between the fingers of the parallel resonators, wherein ts, tds and tdp are related by the following equation: 2ts=

[0016] λ 0,s , and 0.85λ 0,d ≤2tds<2tdp≤1.15λ 0,d , where λ 0,s is the wavelength of the fundamental shear bulk acoustic resonance in the piezoelectric plate, and λ 0,d is the wavelength of the fundamental shear bulk acoustic resonance in the dielectric layer.

[0017] The present invention further relates to a filter device, comprising: a substrate; a lithium niobate piezoelectric plate having a parallel front side and a back side and a thickness of ts, the back side being attached to the substrate; a conductor pattern formed on the front side, the conductor pattern comprising a plurality of interdigital transducers (IDTs) corresponding to a plurality of resonators, the plurality of resonators comprising parallel resonators and series resonators, the interlaced fingers of each of the plurality of IDTs being arranged on a corresponding portion of the piezoelectric plate, the corresponding portion of the piezoelectric plate being suspended above one or more cavities formed in the substrate; a first SiO2 layer having a thickness of tds, deposited between the fingers of the series resonator; and a second SiO2 layer having a thickness of tdp, deposited between the fingers of the parallel resonator, wherein ts, tds and tdp are related by the following formula: 0.875ts≤tds <tdp≤1.25ts。

[0018] Wherein, at the resonant frequency and anti-resonant frequency of all the multiple resonators, the frequency temperature coefficient of the acoustic wave resonator device is between -20ppm / °C and -42ppm / °C.

[0019] The present invention further relates to a method for manufacturing an acoustic wave resonator device on a single crystal piezoelectric plate having parallel front and back sides, the back side being attached to the substrate, the method comprising: forming a cavity in the substrate so that a portion of the single crystal piezoelectric plate forms a diaphragm spanning the cavity; forming an interdigital transducer (IDT) on the front side of the single crystal piezoelectric plate so that the interlaced fingers of the IDT are arranged on the diaphragm, the single crystal piezoelectric plate and the IDT are configured so that a radio frequency signal applied to the IDT excites a shear main acoustic mode in the diaphragm; and forming a half-λ dielectric layer on one of the front and back sides of the piezoelectric plate.

[0020] The thickness of the piezoelectric plate ts and the thickness of the dielectric layer td are defined as follows: 2ts =

[0021] λ 0,s , and 0.85λ 0,d ≤2td≤1.15λ 0,d , where λ 0,s is the wavelength of the fundamental shear bulk acoustic resonance in the piezoelectric plate, and λ 0,d is the wavelength of the fundamental shear bulk acoustic resonance in the dielectric layer.

[0022] Wherein, forming the semi-λ dielectric layer further comprises: depositing one or more of SiO2, Si3N4, Al2O3 and AlN.

[0023] Wherein, the piezoelectric plate is lithium niobate, and forming the half-λ dielectric layer includes depositing SiO2 to a thickness td, wherein td is greater than or equal to 0.875ts and less than or equal to 1.25ts, wherein ts is the thickness of the piezoelectric plate. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Included are a schematic plan view and two schematic cross-sectional views of a laterally excited film bulk acoustic resonator (XBAR).

[0025] Figure 2 yes Figure 1 A partially enlarged schematic cross-sectional view of an XBAR.

[0026] Figure 3 is a partial enlarged schematic cross-sectional view of an improved XBAR including a "half-λ" dielectric layer.

[0027] Figure 4 is a graph comparing the admittance of an XBAR with a half-λ dielectric layer and a conventional XBAR.

[0028] Figure 5 is a cross-sectional view of an XBAR with a half-λ dielectric layer profiled, where the profile represents the stress at the resonant frequency.

[0029] Figure 6 is a graph comparing the admittance of three XBARs with half-λ AlN layers.

[0030] Figure 7 is a graph comparing the admittance of three XBARs with half-λ SiO2 layers.

[0031] Figure 8 is a graph comparing the admittance of three other XBARs with a half-λ SiO2 layer.

[0032] Fig. 9 is a graph of the admittance of an XBAR with an excessively thin half-λ SiO2 layer.

[0033] Fig.10 is a graph of the admittance of an XBAR with an excessively thick half-λ SiO2 layer.

[0034] Fig.11 This is a graph showing the temperature frequency coefficient of XBAR as a function of SiO2 thickness.

[0035] Fig.12 is a schematic circuit diagram and layout of a filter using XBAR.

[0036] Fig.13 is a flow chart of a process for fabricating an XBAR including a half-lambda dielectric layer.

[0037] Throughout the specification, elements appearing in the drawings are assigned three-digit or four-digit reference numerals, of which the two least significant digits are specific to the element and the one or two most significant digits are the number of the element shown first. Elements not described in conjunction with a drawing may be assumed to have the same characteristics and functions as previously described elements having the same reference numeral. DETAILED DESCRIPTION

[0038] Device Description

[0039] Figure 1 A simplified schematic top view and orthogonal cross-sectional view of a laterally excited film bulk acoustic resonator (XBAR) 100 described in U.S. Patent No. 10,491,192 are shown. XBAR resonators such as resonator 100 can be used in a variety of RF filters, including band-stop filters, band-pass filters, duplexers, and multiplexers. XBARs are well suited for use in filters in the communication bands with frequencies above 3 GHz.

[0040] The XBAR 100 consists of a thin film conductor pattern formed on the surface of a piezoelectric plate 110, which has parallel front and back sides 112, 114, respectively. The piezoelectric plate is a thin single crystal layer of a piezoelectric material, such as lithium niobate, lithium tantalate, lanthanum gallium silicate, gallium nitride, or aluminum nitride. The piezoelectric plate is cut so that the orientations of the X, Y, and Z crystal axes are known and consistent relative to the directions of the front and back sides. In the example shown in this patent, the piezoelectric plate can be Z-cut, that is, the Z axis is perpendicular to the front and back sides 112, 114. However, XBARs can be made on piezoelectric plates with other crystal orientations.

[0041] The back side 114 of the piezoelectric plate 110 is attached to the surface of the substrate 120, except for a portion of the piezoelectric plate 110 that is not attached to the surface of the substrate 120, wherein this portion of the piezoelectric plate 110 forms a diaphragm 115 that spans a cavity 140 formed in the substrate. The portion of the piezoelectric plate 110 that spans the cavity is referred to herein as the "diaphragm" 115 because this portion is physically similar to the diaphragm of a microphone. Figure 1 As shown, the diaphragm 115 abuts the remainder of the piezoelectric plate 110 around the entire perimeter 145 of the cavity 140. In this case, "abut" means "contiguously connected without anything else in between."

[0042] The substrate 120 provides mechanical support for the piezoelectric plate 110. The substrate 120 can be, for example, silicon, sapphire, quartz, or some other material, or a combination of these materials. The back side 114 of the piezoelectric plate 110 can be bonded to the substrate 120 using a wafer bonding process. Alternatively, the piezoelectric plate 110 is grown on the substrate 120, or the piezoelectric plate 110 is attached to the substrate in some other way. The piezoelectric plate 110 can be attached directly to the substrate, or can be attached to the substrate 120 via one or more intermediate material layers.

[0043] The conventional meaning of “cavity” is “an empty space within a solid.” The cavity 140 may be a hole that passes completely through the substrate 120 (as shown in the AA and BB cross sections), or may be a groove in the substrate 120. For example, the cavity 140 may be formed by selectively etching the substrate 120 before or after the piezoelectric plate 110 is attached to the substrate 120.

[0044] The conductor pattern of XBAR 100 includes an interdigital transducer (IDT) 130. IDT 130 includes a first plurality of parallel fingers, such as finger 136, extending from a first busbar 132, and a second plurality of fingers extending from a second busbar 134. The first and second plurality of parallel fingers are staggered. The staggered fingers overlap a distance AP, which is generally referred to as the "aperture" of the IDT. The center-to-center distance L between the outermost fingers of IDT 130 is the "length" of the IDT.

[0045] The first and second busbars 132, 134 serve as terminals of the XBAR 100. A radio frequency or microwave signal applied between the two busbars 132, 134 of the IDT 130 excites a primary acoustic mode in the piezoelectric plate 110. The primary acoustic mode is a bulk shear mode, in which acoustic energy propagates in a direction substantially perpendicular to the surface of the piezoelectric plate 110, which is also perpendicular or transverse to the direction of the electric field generated by the IDT fingers. Therefore, the XBAR is considered a transversely excited thin film bulk wave resonator.

[0046] The IDT 130 is placed on the piezoelectric plate 110 such that at least the fingers of the IDT 130 are disposed on a portion 115 of the piezoelectric plate that spans or overhangs the cavity 140. Figure 1 As shown, cavity 140 is rectangular, the size of which is larger than aperture AP and length L of IDT 130. The cavity of XBAR can have different shapes, such as regular or irregular polygons. The cavity of XBAR can have more or less than four sides, which can be straight or curved.

[0047] In order to facilitate Figure 1 1, the geometric spacing and width of the IDT fingers are greatly exaggerated relative to the length (dimension L) and aperture (dimension AP) of the XBAR. A typical XBAR has more than ten parallel fingers in the IDT 110. An XBAR may have hundreds, possibly thousands, of parallel fingers in the IDT 110. Similarly, the thickness of the IDT fingers is greatly exaggerated in the cross-sectional view.

[0048] Figure 2 A detailed schematic cross-sectional view of an XBAR 200 is shown, which may be Figure 1 XBAR 100. Piezoelectric plate 210 is a single crystalline layer of piezoelectric material having a front side 214 and a back side 216. The thickness ts between the front side 214 and the back side 216 may be, for example, 100 nm to 1500 nm. When used for 5G NR (fifth generation new radio) and 3.3 GHz to 6 GHz Wi-Fi TM When the filter has a frequency band, the thickness ts can be, for example, 280 nm to 550 nm.

[0049] The IDT fingers 238 may be aluminum, substantially aluminum alloys, copper, substantially copper alloys, tungsten, molybdenum, beryllium, gold, or some other conductive material. A thin (relative to the total thickness of the conductor) layer of other metals (e.g., chromium or titanium) may be formed under and / or over the fingers to improve adhesion between the fingers and the piezoelectric plate 210 and / or to passivate or encapsulate the fingers. The busbars ( Figure 1 132, 134) can be made of the same or different material as the fingers.

[0050] Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, which may be referred to as the pitch of the IDT and / or the pitch of the XBAR. Dimension w is the width or "mark" of the IDT fingers. The IDT of an XBAR is significantly different from the IDT used in a surface acoustic wave (SAW) resonator. In a SAW resonator, the pitch of the IDT is half the wavelength of the acoustic wave at the resonant frequency. In addition, the mark-to-pitch ratio of a SAW resonator IDT is typically close to 0.5 (i.e., the width of the mark or finger is approximately one-quarter the wavelength of the acoustic wave at resonance). In an XBAR, the pitch p of the IDT is typically 2 to 20 times the width w of the finger. In addition, the pitch p of the IDT is typically 2 to 20 times the thickness ts of the piezoelectric plate 210. The width of the IDT fingers in an XBAR is not limited to one-quarter the wavelength of the acoustic wave at resonance. For example, the width of an XBAR IDT finger may be 500 nm or greater, so that the IDT may be manufactured using photolithography techniques. The thickness tm of the IDT finger may be from 100 nm to approximately equal to the width w. IDT busbar ( Figure 1 The thickness of 132, 134) in the IDT finger can be equal to or greater than the thickness tm of the IDT finger.

[0051] When the thickness ts of the piezoelectric plate is an integer multiple of half the wavelength λ of the acoustic wave, a shear bulk acoustic wave (BAW) propagating perpendicular to the surface of the piezoelectric plate will be reflected from the surface and resonate or form a standing wave. The longest wavelength / lowest frequency at which this resonance occurs is the frequency f0 and the wavelength λ 0,s The shear BAW fundamental resonance at wavelength λ 0,s is equal to twice the thickness of the piezoelectric plate ts. 0,s ” refers to the wavelength of the fundamental (0th order) resonance of the shear BAW in the piezoelectric plate. The wavelength of the same acoustic wave (i.e., a shear BAW propagating in the same direction at the same frequency) may be different in other materials. The frequency f0 can be calculated by dividing the velocity of the shear BAW in the piezoelectric plate by the wavelength λ 0，s The shear BAW fundamental resonance of the piezoelectric plate is different from the resonance of the XBAR device 200, which is affected by the IDT structure.

[0052] Figure 3 A detailed schematic cross-sectional view of an XBAR containing a "half-lambda" dielectric layer is shown. Figure 3 Specifically, the front side of the piezoelectric plate 310 (ie, the side facing away from the substrate; Figure 3 300 having a thick dielectric layer 350 on the upper side (shown). A comparable dielectric layer on the back side 316 of the piezoelectric plate 310 may be used instead of the dielectric layer 350. In greater detail, the XBAR 300 having the thick dielectric layer 350 is similar to Figure 1 XBAR 100. Figure 3Also shown are two IDT fingers 338 as previously described. Dimension p is the center-to-center spacing or "pitch" of the IDT fingers, and dimension w is the width or "mark" of the IDT fingers.

[0053] The piezoelectric plate 310 is a thin single crystal layer of a piezoelectric material such as lithium niobate or lithium tantalate. The piezoelectric plate 310 is cut so that the orientation of the X, Y and Z crystal axes relative to the front and back surfaces 314, 316 is known and consistent. The thickness ts of the piezoelectric plate 310 can be, for example, 100 nm to 1500 nm.

[0054] Dielectric layer 350 can be nearly any dielectric material, such as SiO2, Si3N4, Al2O3, AlN, and other dielectric materials. As will be discussed, particular benefits can occur when the dielectric material is or includes AlN and when the dielectric material is SiO2.

[0055] The thickness ts of the piezoelectric plate 310 and the thickness td of the dielectric layer 350 are configured such that the shear BAW propagating perpendicular to the surfaces 316 and 352 forms a full-period standing wave between the surfaces 316 and 352 at a predetermined frequency, which may be slightly less than the desired resonant frequency of the XBAR device 300. In other words, the shear BAW second overtone resonance occurs at the predetermined frequency. By definition, the thickness ts of the piezoelectric plate is λ 0,s , which, as previously described, is the wavelength of the shear BAW fundamental resonance of the piezoelectric plate 310 without the dielectric layer 350. Nominally, the thickness td of the dielectric layer 350 is λ 0，d One half of 0，d is the wavelength of the same bulk BAW in dielectric layer 350. In this case, each of piezoelectric plate 310 and dielectric layer 350 will contain a half-period standing wave at frequency f0, where frequency f0 is now the frequency of the second overtone resonance. 0，d Equal to λ 0，s , multiplied by the ratio of the shear acoustic wave velocity in the dielectric layer 350 to the shear acoustic wave velocity in the piezoelectric plate 310. For relatively slow dielectric materials, such as SiO2, λ 0,d Can be equal to or slightly larger than λ 0,s In this case, the thickness td of the dielectric layer 350 may be equal to or slightly greater than ts. For relatively fast dielectric materials, such as Si3N4 or AlN, λ0,d may be significantly greater than λ0,s. In this case, the thickness td of the dielectric layer 350 will be proportionally greater than ts.

[0056] Although dielectric layer 350 is referred to herein as a "half-λ" dielectric layer, the thickness td of the dielectric layer need not be exactly λ. 0，d / 2 The thickness td can be different from λ 0，d / 2, as long as the combined thickness of the piezoelectric plate 310 and the dielectric layer 350 is such that the second overtone resonance of the bulk shear wave occurs at a predetermined frequency. Simulation results (some of which are discussed below) show that the dielectric layer thickness results in an XBAR with low spurious modes and consistent electromechanical coupling, where the dielectric layer thickness range is defined by

[0057] 0.85λ 0,d ≤2td≤1.15λ 0,d (1)

[0058] Values ​​of td outside this range result in reduced electromechanical coupling and increased spurious modes. Varying td within this range allows tuning the resonant frequency of the XBAR by approximately 10%, which is sufficient to establish the necessary frequency offset between parallel and series resonators for many filter applications.

[0059] exist Figure 3 , dielectric layer 350 is shown as being deposited over and between IDT fingers 338. In other embodiments, a half-λ dielectric layer may be formed only between IDT fingers. Half-λ dielectric layer 350 may be a single layer or two or more layers of different dielectric materials having similar acoustic impedances.

[0060] The main benefit of incorporating a half-lambda dielectric layer 350 into the XBAR 300 is the increased thickness of the diaphragm. Depending on the material used in the half-lambda dielectric layer 350, the thickness of the diaphragm of the XBAR 300 can be Figure 1 The thickness of the XBAR 100 is two to three times that of the diaphragm 115. The thicker diaphragm is stiffer and less likely to bend or deform with temperature changes.

[0061] The thicker membrane of XBAR 300 will also have a higher thermal conductivity, especially if the half-lambda dielectric layer 350 is or includes a high thermal conductivity dielectric material, such as aluminum nitride. The higher the thermal conductivity, the more efficient the removal of heat from the membrane, thereby requiring less resonator area to be used for a given heat load or power consumption.

[0062] (For the same IDT spacing) and Figure 1 The XBAR 300 will also have a higher capacitance per unit area than the XBAR 100. Resonator capacitance is a circuit design issue. In particular, RF filters using acoustic resonators are often required to meet a requirement that the input and output impedances of the filter match a defined value (usually 50 ohms). This requirement dictates a minimum capacitance value for some or all of the resonators in the filter. The higher capacitance per unit area of ​​the XBAR 300 with partial Bragg reflectors allows a smaller resonator area to be used to meet any desired capacitance value.

[0063] The stiffness and thermal conductivity of an XBAR having a half-lambda dielectric layer on the back side of the piezoelectric plate 310 (not shown) are improved, but the capacitance per unit area is only slightly increased.

[0064] Figure 4 is a graph 400 comparing the admittance of an XBAR having a half-λ dielectric layer and a conventional XBAR. Figure 4 The data shown and subsequently Figure 6 , Figure 7 The data shown are the results of simulating an XBAR device using the finite element method. The solid line 410 is a plot of the admittance magnitude of an XBAR including a half-λ dielectric layer as a function of frequency. The piezoelectric plate is 400 nm thick lithium niobate. The IDT is 100 nm thick aluminum. The spacing and markings of the IDT fingers are 4.25 μm and 1.275 μm, respectively. The half-λ dielectric layer consists of a layer of Si3N4 350 nm thick and a layer of AlN 350 nm thick. The resonant frequency is 4.607 GHz and the antiresonant frequency is 4.862 GHz. The difference between the antiresonant frequency and the resonant frequency is 255 MHz, or about 5.4% of the average of the resonant frequency and the antiresonant frequency.

[0065] Dashed line 420 is a graph of the magnitude of the admittance of a conventional XBAR as a function of frequency. The piezoelectric plate is 400 nm thick lithium niobate. The IDT is 100 nm thick aluminum. The spacing and markings of the IDT fingers are 3.7 μm and 0.47 μm, respectively. The resonant frequency is 4.71 GHz and the anti-resonant frequency is 5.32 GHz. The difference between the anti-resonant frequency and the resonant frequency is 610 MHz, or about 12.2% of the average of the resonant and anti-resonant frequencies. The admittance of the conventional XBAR (dashed line 420) exhibits some parasitic modes between the resonant and anti-resonant frequencies that are not present in the admittance of the XBAR with a half-λ dielectric layer (solid line 410).

[0066] Incorporating a half-lambda dielectric layer in XBAR device 300 results in a stiffer membrane with higher thermal conductivity and potentially fewer excited parasitic modes than conventional XBAR devices. These benefits come at the expense of reduced electromechanical coupling and a corresponding reduction in the difference between the resonant and antiresonant frequencies.

[0067] Figure 5 is a cross-sectional view of an XBAR 500 with a half-λ dielectric layer, where the contours represent the stress at the resonant frequency. The piezoelectric plate 510 is 400 nm thick lithium niobate. The IDT fingers 538 are 100 nm thick aluminum. The half-λ dielectric layer 550 consists of a layer of Si3N4 552 350 nm thick and a layer of AlN 554 350 nm thick.

[0068] The stress in XBAR 500 at the resonant frequency illustrates a full-cycle standing wave between the device surfaces. The stress is highest near the center of the thickness of the piezoelectric plate 510 and near the center of the dielectric layer 550, which corresponds to the peak of the two half-cycle standing waves. The stress is lowest near the boundary between the device surface and the piezoelectric plate 510 and near the center of the dielectric layer 550, corresponding to the zero crossing of the standing wave.

[0069] Figure 6 is a graph 600 illustrating the use of spacing and dielectric thickness to tune the resonant and anti-resonant frequencies of an XBAR having a half-λ dielectric layer. The solid line 610 is a graph of the magnitude of the admittance versus frequency for an XBAR having spacing and markers of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is 400 nm thick lithium niobate. The IDT is 100 nm thick aluminum. The half-λ dielectric layer consists of a 700 nm thick layer of Si3N4. The resonant frequency is 4.513 GHz and the anti-resonant frequency is 4.749 GHz. The difference between the anti-resonant and resonant frequencies is 236 MHz, or about 5.1% of the average of the resonant and anti-resonant frequencies.

[0070] Dotted line 620 is a plot of the admittance magnitude versus frequency for a similar XBAR with the same construction but with the IDT fingers spaced and labeled at 3.75 μm and 1.31 μm, respectively. The resonant frequency is 4.557 GHz and the anti-resonant frequency is 4.795 GHz. Changing the IDT spacing from 4.25 μm to 3.75 μm increases the resonant and anti-resonant frequencies by approximately 45 MHz. Changing the spacing in the range of 3 μm to 5 μm will provide a tuning range of approximately 200 MHz.

[0071] Dashed line 630 is a plot of the magnitude of the XBAR-like admittance as a function of frequency. The spacing and markings of the IDT fingers are 4.25 μm and 1.275 μm, respectively, and the dielectric layers include a 700 nm thick Si3N4 layer and a 50 nm thick SiO2 layer. The resonant frequency is 4.400 GHz and the anti-resonant frequency is 4.626 GHz. Adding a 50 nm "tuning layer" reduces the resonant and anti-resonant frequencies by about 110 MHz.

[0072] Figure 77 is another graph 700 showing the use of spacing and dielectric thickness to tune the resonant and anti-resonant frequencies of an XBAR with a half-λ dielectric layer. The solid line 710 is a graph of the magnitude of the admittance versus frequency for an XBAR with spacing and markers of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is 400 nm thick lithium niobate. The IDT is 100 nm thick aluminum. The half-λ dielectric layer consists of a 400 nm thick layer of SiO2. The resonant frequency is 4.705 GHz and the anti-resonant frequency is 5.108 GHz. The difference between the anti-resonant frequency and the resonant frequency is 403 MHz, or about 8.2% of the average of the resonant and anti-resonant frequencies.

[0073] Dotted line 720 is a plot of the admittance magnitude versus frequency for a similar XBAR with the same construction but with the IDT fingers spaced and labeled at 3.75 μm and 1.31 μm, respectively. The resonant frequency is 4.740 GHz and the anti-resonant frequency is 5.137 GHz. Changing the IDT spacing from 4.25 μm to 3.75 μm increases the resonant and anti-resonant frequencies by approximately 35 MHz. Changing the spacing in the range of 3 μm to 5 μm will provide a tuning range of approximately 100 MHz.

[0074] The dashed line 730 is a plot of the magnitude of the admittance versus frequency for a similar XBAR of the same construction, except that the IDT finger spacing and marks are 4.25 μm and 1.275 μm, respectively, and the dielectric layer is 450 nm thick SiO2. The resonant frequency is 4.512 GHz and the anti-resonant frequency is 4.905 GHz. The difference between the anti-resonant frequency and the resonant frequency is 393 MHz, or about 8.3% of the average of the resonant and anti-resonant frequencies. Increasing the thickness of the dielectric layer by 50 nm would reduce the resonant and anti-resonant frequencies by about 190 MHz without reducing the electromechanical coupling.

[0075] Figure 8 is another graph 800 showing the use of dielectric layer thickness to tune the resonant and anti-resonant frequencies of an XBAR having a half-λ dielectric layer. Dotted line 810 is a graph of the magnitude of the admittance versus frequency for an XBAR having a spacing and marker of 4.25 μm and 1.275 μm, respectively. The piezoelectric plate is 400 nm thick lithium niobate. The IDT is 100 nm thick aluminum. The half-λ dielectric layer consists of a layer of SiO2 425 nm thick. This example shows that the thickness of the dielectric layer td is equal to λ 0,d / 2 situation.

[0076] The solid line 820 is a graph of the admittance magnitude versus frequency for a similar XBAR of the same construction, except that the XBAR here has a dielectric layer of 375 nm thick SiO2. In this case, td = 0.88 (λ 0,d / 2). Dashed line 830 is a graph of the admittance magnitude versus frequency for a similar XBAR of the same construction, except that the XBAR here has a dielectric layer of 475 nm thick SiO2. In this case, td = 1.12 (λ 0,d / 2 ). Changing SiO2 from 375nm to 475nm shifts the resonant and anti-resonant frequencies by about 400MHz while maintaining electromechanical coupling and not introducing troublesome parasitic modes.

[0077] Assuming a 400nm thick lithium niobate piezoelectric plate, the range of td expressed in equation (1) corresponds to about 350nm to 500nm. The thickness ts of the piezoelectric plate can be expressed in this range:

[0078] 0.875 ts≤td≤1.25 ts (2)

[0079] It is expected that this range will apply to any thickness of the lithium niobate piezoelectric plate.

[0080] Fig. 9 is a graph 900 illustrating the effect of a too thin "half-lambda" dielectric layer. The solid line 910 is a graph with Figure 8 The XBAR with the same structure as the device is shown in the graph of the admittance size versus frequency. The thickness of the SiO2 dielectric layer is reduced to 325nm. In this case, td = 0.76 (λ 0,d / 2 ). Reducing the dielectric thickness to a point where electromechanical coupling is reduced and very large parasitic modes occur below the device resonant frequency.

[0081] Fig.10 is a graph 1000 illustrating the effect of a too thick "half-lambda" dielectric layer. The solid line 910 is a graph with Figure 8 The XBAR with the same structure as the device is shown in the graph of the admittance size versus frequency. The thickness of the SiO2 dielectric layer is increased to 525nm. In this case, td = 1.24 (λ 0,d / 2 ). Increasing the dielectric thickness to the point where electromechanical coupling is reduced and very large parasitic modes occur above the device resonant frequency.

[0082] The frequency temperature coefficient of SiO2 and the frequency temperature coefficient of lithium niobate have similar magnitudes and opposite signs. Compared with conventional XBAR devices, the frequency variation of XBAR devices with SiO2 semi-λ dielectric layers over temperature is much smaller.

[0083] Fig.11 is a graph of the frequency temperature coefficient of XBAR as a function of SiO2 thickness. Specifically, the solid line 1110 is the admittance characteristic previously described in Figure 7 and Figure 811. The temperature coefficient of the anti-resonance frequency of the XBAR device shown in FIG. The dashed line 1120 is the temperature coefficient of the resonant frequency of the same device. The simulation results show that the frequency temperature coefficient of the conventional XBAR device without a dielectric layer is about -113ppm / C0. The presence of the SiO2 half-λ dielectric layer reduces the magnitude of the frequency temperature coefficient by about 3 times.

[0084] Fig.12 1 is a schematic circuit diagram of a bandpass filter 1200 using five XBARs X1-X5. The filter 1200 may be, for example, an n79 bandpass filter used in communication devices. The filter 1200 has a conventional ladder filter architecture, including three series resonators X1, X3, X5 and two parallel resonators X2, X4. The three series resonators X1, X3, X5 are connected in series between a first port and a second port. Fig.12 In FIG. 1 , the first port and the second port are labeled "In" and "Out", respectively. However, the filter 1200 is bidirectional and either port can be used as the input or output of the filter. The two parallel resonators X2 and X4 are connected to ground from the node between the series resonators. All parallel resonators and series resonators are XBARs.

[0085] The three series resonators X1, X3, X5 and the two parallel resonators X2, X4 of the filter 1200 can be formed on a single plate 1230 of piezoelectric material bonded to a silicon substrate (not visible). Each resonator includes a corresponding IDT (not shown), at least the fingers of the IDT are arranged above the cavity in the substrate. In this and similar contexts, the term "corresponding" means "associating things with each other", i.e., one-to-one correspondence. Fig.12 , the cavity is schematically illustrated as a dashed rectangle (e.g., rectangle 1235). In this example, the IDT of each resonator is arranged above the corresponding cavity. In other filters, the IDTs of two or more resonators can be arranged on a common cavity. The resonators can also be cascaded into multiple IDTs, which can be formed on multiple cavities.

[0086] Each of the resonators X1 to X5 has a resonant frequency and an anti-resonant frequency. In short, each resonator is actually a short circuit at its resonant frequency and an open circuit at its anti-resonant frequency. Each resonator X1 to X5 creates a "zero transmission" where the transmission between the input and output ports of the filter is very low. Note that the transmission at the "zero transmission" is not actually zero due to energy leakage through parasitic components and other effects. The three series resonators X1, X3, and X5 produce zero transmission at their respective anti-resonant frequencies (where each resonator is actually an open circuit). The two parallel resonators X2 and X4 produce zero transmission at their respective resonant frequencies (where each resonator is actually a short circuit). In a typical bandpass filter using acoustic resonators, the anti-resonant frequency of the series resonator is higher than the upper edge of the passband, so that the series resonator produces zero transmission above the passband. The resonant frequency of the parallel resonator is less than the lower edge of the passband, so that the parallel resonator produces zero transmission below the passband.

[0087] refer to Figure 7 and Figure 8 From the data of the relationship between admittance and frequency, it can be seen that the frequency offset between the resonant frequency and the anti-resonant frequency of the XBAR with a 400nm lithium niobate piezoelectric plate and a half-λ dielectric layer is about 400MHz. For bandpass filters in telecommunication bands such as n79 (4400MHz to 5000MHz) and n77 (3300MHZ to 4200MHz), this frequency separation itself is insufficient. U.S. Patent No. 10,491,192 describes the use of a dielectric layer deposited on a parallel resonator to reduce the resonant frequency of the parallel resonator relative to the resonant frequency of the series resonator. Patent No. 10,491,192 describes a filter of the following characteristics, wherein there is a very thin dielectric layer or no dielectric layer on the series resonator and the thickness of the dielectric layer is about 0.25 times the thickness of the piezoelectric plate on the parallel resonator.

[0088] When the resonator is an XBAR with a half-λ dielectric layer, a similar approach can be used to reduce the resonant frequency of the parallel resonator relative to the series resonator. In this case, the thickness of the dielectric layer on the series resonator, tds, and the thickness of the dielectric layer on the parallel (shunt) resonator, tdp, can be defined as

[0089] 0.85 λ 0,d ≤2tds<2tdp≤1.15λ 0,d (3)

[0090] Reference Figure 8, the solid line 820 is the admittance of the XBAR with a 375nm SiO2 layer on a 400nm lithium niobate piezoelectric plate. The dashed line 830 is the admittance of the XBAR with a 475nm SiO2 layer on a 400nm lithium niobate piezoelectric plate. A filter such as filter 1200 can be made by using a 400nm lithium niobate piezoelectric plate with a 375nm SiO2 layer on a series resonator and a 400nm lithium niobate piezoelectric plate with a 475nm SiO2 layer on a parallel resonator. In this case, the frequency spacing between the resonant frequency of the parallel resonator and the anti-resonant frequency of the series resonator is about 800MHz, which is sufficient for a bandpass filter in the n79 band. The frequency spacing will be proportional to the thickness of the piezoelectric plate.

[0091] The thickness range of the SiO2 layer on the series and parallel resonators can be expressed in terms of the thickness ts of the lithium niobate piezoelectric plate as follows:

[0092] 0.875 ts≤tds <tdp≤1.25 ts, (4)

[0093] Where tds is the thickness of the SiO2 layer on the series resonator, and tdp is the thickness of the SiO2 layer on the shunt (parallel) resonator.

[0094] Method Description

[0095] Fig.13 1 is a simplified flow chart showing a method 1300 for making an XBAR including a partial Bragg reflector or a filter including such an XBAR. The method 1300 begins at 1305, where a thin piezoelectric plate is disposed on a sacrificial substrate 1302 and a device substrate 1304. The method 1300 ends at 1395, where the XBAR or filter is completed. Fig.13 The flowchart only includes the main processing steps. Fig.13 Various conventional processing steps (eg, surface preparation, cleaning, inspection, baking, annealing, monitoring, testing, etc.) are performed before, between, after, and during the steps shown.

[0096] Fig.13 Three variations of method 1300 for making an XBAR are captured, which differ in when and how the cavity is formed in the substrate. The cavity can be formed in steps 1310A, 1310B, or 1310C. Only one of these steps is performed in each of the three variations of method 1300.

[0097] Thin plates of single crystal piezoelectric material bonded to non-piezoelectric substrates are commercially available. At the time of this application, both lithium niobate and lithium tantalate plates are available for bonding to a variety of substrates, including silicon, quartz, and fused quartz. Thin plates of other piezoelectric materials may be available now or in the future. The thickness of the piezoelectric plate can be between 300nm and 1000nm. The piezoelectric plate can be, for example, a Z-cut, a rotated Z-cut, or a rotated Y-cut lithium niobate or lithium tantalate. The piezoelectric plate can be some other material and / or some other cut. The substrate can be silicon. When the substrate is silicon, a layer of SiO2 can be provided between the piezoelectric plate and the substrate. The substrate can be some other material that allows deep cavities to be formed by etching or other processing.

[0098] In one variation of method 1300, one or more cavities are formed in the substrate at 1310A before the piezoelectric plate is bonded to the substrate at 1330. A separate cavity may be formed for each resonator in the filter device. The one or more cavities may be formed using conventional photolithography and etching techniques. For example, deep reactive ion etching (DRIE) may be used to form the cavity. Typically, the cavity formed at 1310A does not penetrate the substrate.

[0099] At 1330, the piezoelectric plates on the sacrificial substrate 1302 and the device substrate 1304 may be bonded. The sacrificial substrate 1302 and the piezoelectric plates on the device substrate 1304 may be bonded using a wafer bonding process, such as direct bonding, surface activated or plasma activated bonding, electrostatic bonding, or some other bonding technique. Prior to the wafer bonding process, the device substrate may be coated with a bonding layer, which may be SiO2 or some other material.

[0100] After the sacrificial substrate 1302 and the piezoelectric plate on the device substrate 1304 are bonded, the sacrificial substrate and any intermediate layers are removed at 1340 to expose the surface of the piezoelectric plate (the surface that previously faced the sacrificial substrate). For example, the sacrificial substrate can be removed by wet or dry etching or some other process depending on the material. The exposed surface of the piezoelectric plate can be polished or processed in some other way at 1340 to prepare the surface and control the thickness of the piezoelectric plate.

[0101] Conductor patterns and dielectric layers defining one or more XBAR devices are formed at 1350. Typically, the filter device will have two or more conductor layers deposited and patterned in sequence. The conductor layer may include pads, gold or solder bumps, or other devices for establishing a connection between the device and an external circuit. The conductor layer may be, for example, aluminum, an aluminum alloy, copper, a copper alloy, molybdenum, tungsten, beryllium, gold, or some other conductive metal. Optionally, one or more layers of other materials may be disposed below the conductor layer (i.e., between the conductor layer and the piezoelectric plate) and / or above the conductor layer. For example, a thin film of titanium, chromium, or other metal may be used to improve the adhesion between the conductor layer and the piezoelectric plate. The conductor layer may include pads, gold or solder bumps, or other devices for establishing a connection between the device and an external circuit.

[0102] A conductor pattern may be formed at 1350 by depositing a conductor layer on the surface of the piezoelectric plate and removing excess metal by etching through the patterned photoresist. Alternatively, a lift-off process may be used to form the conductor pattern at 1350. A photoresist may be deposited on the piezoelectric plate and patterned to define the conductor pattern. Conductor layers may be sequentially deposited on the surface of the piezoelectric plate. The photoresist may then be removed to remove excess material, leaving the conductor pattern.

[0103] At 1360, a semi-λ dielectric layer may be formed on the front side of the piezoelectric layer. The semi-λ dielectric layer may be deposited on the conductor pattern or may be formed only between the fingers of the IDT. In some filter devices, a first dielectric layer may be deposited on / between the fingers of all IDTs, and a second dielectric layer may be selectively formed on a portion of the IDTs, such as only on the IDTs of the parallel resonator. The first dielectric layer is typically thicker than the second dielectric layer. The first dielectric layer and the second dielectric layer may be the same or different materials. Either the first dielectric layer or the second dielectric layer may be deposited first.

[0104] In a second variation of process 1300, after all conductor patterns and dielectric layers are formed at 1350 and 1360, one or more cavities are formed on the back side of the substrate at 1310B. A separate cavity may be formed for each resonator in the filter device. The one or more cavities may be formed using anisotropic or orientation-dependent dry or wet etching to open a hole through the back side of the substrate to the piezoelectric plate.

[0105] In a third variation of process 1300, one or more cavities in the form of grooves can be formed in the substrate at 1310C by etching the substrate using an etchant introduced by the openings in the piezoelectric plate and the semi-lambda dielectric layer. A separate cavity can be formed for each resonator in the filter device. The one or more cavities formed at 1310C do not penetrate the substrate.

[0106] In all variations of process 1300, the filter device is completed at 1370. Actions that may occur at 1370 include depositing an encapsulation / passivation layer such as SiO2 or Si3O4 on all or part of the device and / or forming pads or solder bumps or other means for establishing a connection between the device and an external circuit if these steps were not performed at 1350. Other actions at 1370 may include cutting a single device from a wafer containing multiple devices, other packaging steps, and testing. Another action that may occur at 1370 is tuning the resonant frequency of a resonator within the device by adding or removing metal or dielectric material on the front side of the device. Once the filter device is completed, the process ends at 1395.

[0107] A variation of process 1300 begins with a single crystal piezoelectric wafer at 1302, rather than a thin piezoelectric plate on a sacrificial substrate of a different material. Ions are implanted to a controlled depth ( Fig.13 ). The portion of the wafer from the surface to the depth of the ion implantation is (or will become) a thin piezoelectric plate, and the remainder of the wafer is a sacrificial substrate. As previously described, the piezoelectric wafer and the device substrate are bonded at 1330. At 1340, the piezoelectric wafer can be split at the plane of the implanted ions (e.g., using thermal shock), exposing the thin plate of piezoelectric material and bonding it to the acoustic Bragg reflector. The thickness of the thin plate piezoelectric material is determined in part by the energy (and depth) of the implanted ions. The process of ion implantation and subsequent separation of the thin plate is often referred to as "ion slicing." After the piezoelectric wafer is split, the exposed surface of the piezoelectric plate can be flattened and its thickness can be reduced using, for example, chemical mechanical polishing.

[0108] Conclusion

[0109] Throughout the specification, the embodiments and examples shown should be considered as examples, rather than limitations on the disclosed or required devices and processes. Although many of the examples provided herein relate to specific combinations of method actions or system elements, it should be understood that those actions and those elements can be combined in other ways to achieve the same goals. With respect to the flow charts, additional steps and fewer steps can be taken, and the steps shown can be combined or further refined to achieve the methods described herein. The actions, elements, and features discussed only in conjunction with one embodiment are not intended to exclude similar effects in other embodiments.

[0110] As used herein, "plurality" refers to two or more. As used herein, a "group" of items may include one or more such items. As used herein, whether in a written embodiment or in a claim, the terms "include", "comprising", "carrying", "having", "containing", "involving", etc. should be understood as open-ended, that is, referring to including but not limited to. With respect to the claims, only the transition phrases "consisting of..." and "consisting essentially of..." are closed or semi-closed transition phrases. Ordinal numbers used in the claims, such as "first", "second", "third", etc., are used to modify claim elements, which themselves do not indicate the priority of one claim element over another claim element, or the order, or the order of execution of method actions, but are only used to distinguish one claim element with the same name from another element with the same name (but with ordinal numbers), thereby distinguishing claim elements. As used herein, "and / or" means that the listed items are alternatives, but the alternatives also include any combination of the listed items.

Claims

1. An acoustic resonator device, comprising: substrate; a piezoelectric plate attached to the substrate directly or through one or more intermediate layers; an interdigital transducer, on the piezoelectric plate, such that interdigital fingers of the interdigital transducer are disposed on the piezoelectric plate; as well as A dielectric layer on the piezoelectric plate; Wherein, the thickness ts of the piezoelectric plate and the thickness td of the dielectric layer are defined as follows: 2ts=λ 0,s ,and 0.85l 0,d ≤2td≤1.15λ 0,d , Among them, λ 0,s is the wavelength of the bulk acoustic wave resonance in the piezoelectric plate.

2. The acoustic resonator device according to claim 1, wherein: A portion of the piezoelectric plate is over the cavity of the acoustic resonator device.

3. The acoustic resonator device of claim 1, further comprising an acoustic reflector located between the piezoelectric plate and the substrate.

4. The acoustic resonator device according to claim 1, wherein: The dielectric layer is composed of multiple layers of dielectric materials.

5. The acoustic resonator device according to claim 4, wherein: The frequency temperature coefficient of the acoustic resonator device is between -32 ppm / °C and -42 ppm / °C at the resonant frequency and between -20 ppm / °C and -36 ppm / °C at the anti-resonant frequency.

6. A filter device comprising: substrate; a piezoelectric plate having a thickness ts, the piezoelectric plate being attached to the substrate via one or more intermediate layers; Bulk acoustic series resonator, comprising: a conductor pattern on the piezoelectric plate, the conductor pattern including an interdigital transducer of the bulk acoustic series resonator; and a first dielectric layer having a thickness tds disposed between the interleaved fingers of the IDT of the BAS resonator; and Bulk acoustic parallel resonator, comprising: a conductor pattern on the piezoelectric plate, the conductor pattern including an interdigital transducer of the bulk acoustic parallel resonator; and a second dielectric layer, having a thickness tdp, deposited between the interleaved fingers of the IDT of the BAR, Among them, ts, tds and tdp are related by the following formula: 0.85l 0,d ≤2tds<2tdp≤1.15λ 0,d where λ 0,s is the wavelength of the bulk acoustic wave resonance in the piezoelectric plate, and λ 0,d is the wavelength of bulk acoustic wave resonance in at least one of the first dielectric layer and the second dielectric layer.

7. The filter device according to claim 6, wherein: A portion of the piezoelectric plate is over the cavity of the bulk acoustic series resonator.

8. The filter device of claim 6, further comprising an acoustic reflector between the piezoelectric plate and the substrate.

9. The filter device according to claim 6, wherein: At least one of the first dielectric layer and the second dielectric layer is composed of multiple layers of dielectric materials.

10. The filter device according to claim 6, wherein: The frequency temperature coefficient of the acoustic resonator device is between -32 ppm / °C and -42 ppm / °C at the resonant frequency and between -20 ppm / °C and -36 ppm / °C at the anti-resonant frequency.

11. The filter device according to claim 6, wherein: The bulk acoustic wave resonance has an electric field excited laterally in the piezoelectric plate, and the propagation of the bulk acoustic wave is perpendicular to the lateral excitation of the electric field and substantially perpendicular to the surface of the piezoelectric plate.

12. A filter device, comprising: Bulk acoustic wave parallel resonator, comprising: substrate; a piezoelectric layer having a front side and a back side and a thickness ts, the back side being attached to the substrate; a conductor pattern, on the front surface of the piezoelectric plate, comprising a plurality of IDT fingers; and a dielectric layer having a thickness of tdp deposited between the fingers of the parallel resonator; and Bulk acoustic wave series resonator, comprising: substrate; a piezoelectric layer having a front side and a back side and a thickness ts, the back side being attached to the substrate; a conductor pattern, on the front surface of the piezoelectric plate, comprising a plurality of IDT fingers; and A dielectric layer having a thickness of tds is deposited between the fingers of the series resonator, wherein ts, tds and tdp are related by: 0.875ts≤tds <tdp≤1.25ts。

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